This invention relates to computerized tomographic x-ray sources and in particular to such sources in which the x-ray beam direction is electronically controlled.
In a computerized tomographic imaging system, a beam of x-rays, often having a planar fan shape, is directed through the object under study. Various portions of the body absorb x-ray energy to a greater or lesser degree depending upon a number associated with each point in the body called the coefficient of x-ray absorption. Present day tomographic scanners typically exhibit a resolution in which such "point" is a square approximately 1 mm on a side. To gather sufficient information to determine the coefficient of absorption at points within the body, the x-ray beam is projected through the body from a plurality of different views. These views are typically spaced at regular angular increments in a plane. This plane determines the slice through the body for which an image is generated. This image is actually a pictorial representation of the x-ray coefficient of absorption associated with points in the body. In the resulting picture which is typically displayed on a cathode ray tube, the differing values of these coefficients are associated with different levels on a gray scale and/or with different colors to produce a false-color image.
Early computerized tomography scanners were only used for head scans because of their slow speed. Because the cranial organs undergo minimal movements, their motions posed no problem for these scanners. However, because of the great medical diagnostic advantages offered by computerized tomographic x-ray images, scans through other bodily organs are desired, and in particular scans through moving bodily organs, such as the heart, are highly desirable. For example, such heart scans are useful for determining the effectiveness of coronary artery bypass surgery. Because of the relatively rapid movement of the organs of the thorax and abdomen, it is desirable to collect absorption and data from several hundred views in less than 1 second. At present, relatively high speed computerized tomographic scanning is accomplished by disposing one or more conventional x-ray tubes along a circular rotating gantry which revolves about the patient at speeds of less 1 revolution per second. The radius about which these tubes revolve is approximately 1 meter and is determined essentially by human dimensional constraints. Because of this relatively large radius and because of the desire to have a rotation speed of approximately 1 revolution per second or less, unacceptably large g forces are exerted on the rotating x-ray source which itself often contains, for cooling purposes, a rotating anode.
To avoid the difficulties associated with mechanical rotation of the x-ray source, certain tomography systems employ an electronically scanned electron beam in order to allow much faster movement of the x-ray source point. Examples of certain features of such electronically scanned systems are found, for example, in U.S. Pat. No. 4,122,346 issued Oct. 24, 1978 to H. Enge and in U.S. Pat. No. 4,130,759 issued Dec. 19, 1978 to J. Haimson. A common feature of these systems is the relatively long distance between the electron gun source and the anode target. Because an electron beam comprises, by definition, particles which exhibit the same electrical charge, there is a natural tendency for the electron beam to openly diverge due to space charge forces. If the electron beam divergence is not controlled, insufficient electron beam energy arrives at the anode target. Moreover, the electron beam must be passed through bending coils which produce further aberrations from a convergent beam. Not only must the beam be non-divergent for proper bending and focusing, but the cross section of the electron beam should optimally be circular, that is, the beam should have cylindrical symmetry. In contrast, however, it is highly desirable that the electron beam cross section immediately prior to impingement upon the anode, be rectangular with the long dimension of the rectangle pointing toward the system axis. The rectangular beam cross section at this point is desirable for two reasons. First, because of the typical angle of impingement with the anode target, the cross section of the resultant x-ray beam source can be made to appear square, as viewed from the body or object under study. Second, an electron beam with a rectangular cross section distributes its energy more uniformly across the face of the anode target. Thus, by using a rectangular focal spot, a higher beam wattage is permissible without anode overheating, and the effective focal spot size (which causes loss of image spatial resolution if too large) is no larger than that of a square focal spot. However, if the electron beam, as emitted from an electron gun, were to have such a rectangular cross section, conventional focusing and bending coils would not properly function to produce the desired rectangular focal spot on the anode target.